The efficiency and quality of droplet surfacing depends to a great extent on the rate of electrode melting and on a method used for controlling the detachment of droplets from the electrode and transferring said droplets onto the surface of an article being surfaced.
The problem of controlling the process of droplet detachment and the droplet transfer onto the surface of the article is of a special importance in forming the contacts on contact holders.
Known in the art methods of forming contacts on the contact holders have a number of disadvantages. Thus, a widely spread method of forming the contacts by rivetting contact plates onto the contact holders does not provide for a high stability of contact resistance since in the course of operation the riveted contacts may get loose which results in that the area of contact between the contact plate and the contact holder changes.
Furthermore, the above method is a highly labour-consuming one and causes an elevated consumption of precious metals such as platinum, gold and silver used for manufacturing contact plates. In some cases the loss in weight of a precious metal in the process of cutting-out the contact plate and securing the latter to a contact holder by rivetting exceeds by 25-40% the weight of the contact plate proper.
Soldering the contact plates onto contact holders provides for a higher reliability of the contact connection, however said reliability being ensured only at normal temperatures. At elevated temperatures soldered contacts are not reliable in operation. This method is also a labour-consuming one. In addition, soldering contacts does not practically lend itself to be readily automated, and has therefore a relatively low efficiency.
For fabricating electric contacts use is also made of a method incorporating fixing the contact plates onto contact holders by means of resistance welding, and in particular spot welding. This method feature a higher efficiency and readily lend itself to automation, but does not provide for a high strength and reliability of the joint weld between the contact plate and the contact holder, since the area of the weld spot is much smaller than that of the contact plate. Moreover, quality control of the joint weld produced by this method is difficult to effect.
At present the most efficient technique of forming the electric contacts on contact holders is a process including electric-arc surfacing which in comparison with the above-mentioned methods makes it possible to considerably enhance the strength and reliability of the contact joint, and the stability of the contact resistance, to reduce the consumption of precious materials, and to fully automatize the process.
There is known a method of electric-arc consumable-electrode droplet surfacing /USSR Author's Certificate No. 260,768, 1970/ which comprises feeding a consumable electrode through a current contact tip with a constant rate towards the surface of an article being surfaced, igniting arc between the electrode and the surface of the article so as to form a droplet on the electrode end, moving the tip relative the electrode in the direction opposite to the direction of feeding of the electrode in order to cause detachment of the molten droplet onto said surface of the article. The magnitude of the arc ignition current, the electrode feed rate, and the distance between the end of the current contact tip and the surface of the article are selected so that the molten droplet forms at the end face of the current contact tip.
At the beginning of the electrode melting the rate of melting thereof is higher than the rate of feeding of the electrode, as a result of which the molten droplet approaches the nozzle end and cools down due to the fact that the current contact nozzle serves as a heat removal means. Partially, removal of heat through the current contact nozzle causes the rate of electrode melting to slow down.
The rate of electrode melting become equal to the rate of feeding of the electrode, and the molten droplet all the time is at the end of the nozzle. As the electrode is advanced the size of the droplet continuously increases, and after the molten droplet acquires a required mass the nozzle is removed from the droplet in the direction opposite to the direction of feeding of the electrode. As a result, the droplet ceases cooling down, its temperature sharply rises, the resistance to its detachment decreases and the droplet falls down onto the surface of the article.
An apparatus for carrying out the above method comprises a means for feeding a consumable electrode with a constant speed in the direction of the surface of an article being surfaced, a current contact nozzle, and a drive for reciprocatingly moving the current contact nozzle relative the electrode /"Novaya tekhnologiya, kompleksnaya mekhanizatsiya i avtomatizatsiya svarochnogo proizvodstva" 1977, Kiev, "Tekhnika", Tarasov N. M., Slesarev B. A., Khristoforov B. M. "Argonno-dugovaya naplavka electricheskikh kontactov", pp. 62-65/.
The above method and apparatus for carrying out this method allow stability of the mass of the droplets to be improved only if the droplet mass is close to a value at which there occur non-controllable /spontaneous/ detachment of the droplets.
By the expression "the mass of droplet for non-controllable detachment" is meant a mass at which the gravity force of the molten droplet exceeds the sum of the forces retaining the droplet at the end of the electrode.
It should be noted that such a method and apparatus do not provide for the detachment of the droplets having a mass which is less than the mass required for non-controllable detachment, which considerably limits a controllable range of the mass of the droplets being detached.
There is also known a method of electric-arc consumable electrode surfacing by droplets /cf. CzSR Pat. No. 116,410, Int. Cl..sup.2 B 23K 9/12, 1970/ which comprises feeding a consumable electrode through a current contact nozzle towards the surface of an article being surfaced, igniting an electric spark between the electrode and the surface of the article to form on the electrode tip a molten droplet, translatory moving the electrode with the droplet thereon towards the surface with a speed at which the kinetic energy of the molten droplet does not exceed the energy required for the detachment of the droplet from the electrode, and subsequently causing the droplet to detach and fall down onto the surface of the article, which is effected by imparting to the electrode together with the droplet accelerating motion in the direction opposite to the direction of the electrode feeding.
The above method is carried out with the use of a known in the art apparatus /cf. USSR Author's Certificate No. 526,468, Int. Cl. B 23K 9/12, 1976/ comprising a means for feeding a consumable electrode to the surface of the article being surfaced, and a current contact tip fixed at a predetermined distance from the surface of said article. The means for feeding the electrode includes a cam disk determining the electrode feed speed, connected with a rotational drive and an electrode holder mounted for reciprocal movement relative the current contact tip. The cam disk profile is formed by an intermittently decreasing radius. A spring mounted between the electrode holder and the current contact tip continuously maintains a contact between the electrode holding means and the cam disk.
In operation the cam disk, while rotating, impart a translatory motion to the electrode holder together with the electrode clamped thereby towards the surface of the article being surfaced. At the same moment a spark is ignited between the electrode and said surface, and as a result of sparking a molten droplet is formed on the tip of the electrode.
At the moment when the cam disk acts with its notch upon the electrode holder there occur a change in the movement of the electrode holder. Said holder under the action of the spring begins to acceleratingly move in the direction opposite to the direction of feeding of the electrode, which results in the detachment of the molten droplet and forcing said droplet to fall down onto the surface of the article, the acceleration of this movement is determined by the spring characteristic.
The method and the apparatus described above enable controlling the mass of the molten droplets in a wide range, and thereby provide forming droplets having a mass being considerably smaller than the mass of the droplets in the case of non-controllable detachment.
However, the above method and apparatus do not provide for a high accuracy of transfer of droplets onto the surface of an article being surfaced due to the fact that at the moment of detachment of the drops from the electrode, while the latter is moving in the direction opposite to the direction of feeding of the electrode, the said drop also receive a motive pulse causing it to move in the same direction as the electrode does, which distorts the droplet falling path towards the surface of the article. Moreover, when in use they do not always ensure the required mass of droplets since at the moment of detachment of the droplet a portion thereof remains on the electrode.
The USSR Author's Certificate No. 453,008 /Int. Cl. B 23K 9/12, 1976/ discloses a method of electric-arc gas shielded surfacing by molten droplets with consumable electrodes which comprises the steps of feeding a consumable electrode through a current contact tip to the surface of an article being surfaced, igniting electric arc between the electrode and the surface of the article to form a molten droplet on the electrode end, feeding a shielding gas into the electrode melting zone, and subsequently causing said tip to translatory move towards the surface of the article. The tip while moving in said direction relative the electrode shoves the droplet, thereby throwing it down onto the surface of the article, whereafter the tip moves back to its initial position. By varying the speed of movement of the tip relative the electrode the speed of movement of the molten droplets towards the surface of the article may be also varied. Varying the electrode feed rate and presetting the frequency of impacts of the tip enable the mass of molten droplets to be varied in a wide range.
The above-described method is practiced with the use of an apparatus comprising a means for feeding a consumable electrode towards the surface of an article being surfaced, and a body wherein is mounted a nozzle provided with an inlet pipe for feeding a shielding gas into a cavity of said nozzle and having an opening adapted for directing said gas into the electrode melting zone /cf. "Povyshenie proizvoditelnosti i katchestva naplavochnyh rabot pri remonte i isgotovlenii detalei mashin i mekhanizmov", published in Moscow in 1977 (Moskva, Moskovskiy dom nauchno-tehnicheskoi propagandy im. F. E. Dzerdzinskogo): N. M. Tarasov i B. A. Slesarev. "Elektrodugovaya naplavka elektricheskih kontaktov", see pp. 141-146, and in particular p. 142/.
The above apparatus includes a current contact tip mounted inside the nozzle and provided with a hole for the electrode to pass through, which current contact tip is connected to a reciprocating drive capable of imparting to said tip on accelerating motion relative the nozzle in the direction of the surface of the article being surfaced. The tip is provided with a return spring.
The method and the device described above ensure a high degree of accuracy of transfer of the molten droplet onto the surface of the article. However, at the moment when the droplet is detached by the tip, a portion of the molten droplet which contacts the tip is cooled down with the result that the viscosity of said portion increases, which leads to the result that this portion of the droplet is retained on the electrode, thereby lowering the stability and accuracy of the droplet mass control in the course of surfacing.
Furthermore, this method ensures reliable detachment of the droplet by said current contact tip only when the diameter of the droplet is at least 1.5-2 times exceeds the diameter of the electrode. Otherwise, the tip moving along the electrode by-passes the droplet without detaching it. Therefore the range of modifying the droplet mass is limited.